Variable-reluctance transducers have been commercially available for many years, and typical known configurations are shown in my U.S. Pat. Nos. 3,118,121, 3,140,450 and 3,562,682 which are directed to pressure measuring instruments. The basic variable-reluctance sensing mechanism, however, measures movement or displacement, and is accordingly adaptable to other kinds of transducers such as accelerometers, vibrometers, and the like. When used in a pressure transducer, a force-summing linearly reacting diaphragm is exposed to the fluid pressure to be measured, and the resulting deflection of the diaphragm displaces the elements of the variable-reluctance system (typically in a bridge configuration) to generate an electrical signal proportional to movement, and hence to pressure. Other force-to-motion converters can also be used, such as the well-known Bourdon tube.
In a typical two-active-arm oscillator-excited Wheatstone-bridge configuration, the variable-reluctance sensor includes a pair of spaced-apart coils, each wound around the center leg of an E-shaped high-permeability core of iron or a similar magnetic material, there being an air gap between the opposed coil-core sets. A magnetic element or armature in the air gap is moved or displaced by the force sensor (e.g., diaphragm, seismic mass, etc.), thereby varying the reluctance of the magnetic circuit by increasing the air gap for one coil, and proportionately decreasing the air gap for the other. This results in proportional and opposite changes of the coil inductances, enabling generation of an electrical signal directly related to the phenomenon (pressure, etc.) being measured.
Variable-reluctance sensors are rugged and generate relatively high output signals (in the order of volts rather than millivolts) as compared to other kinds of sensors such as strain gages. The large output signal simplifies the overall instrumentation system, and provides a good signal-to-noise ratio relative to the noise inherent in the associated electronic circuitry (oscillator, demodulator, and the like). There is nevertheless a problem with these and similar devices when the objective is to measure minute variations of the measured parameter while that parameter is at a high level which is close to or at the full-scale limit of the transducer.
For example, there are pressure-measuring applications where the average pressure level is high (e.g., 250 psi), but there is a need for measuring very small variations (e.g., one-tenth inch of water, or about 0.0036 psi) from this average level. A high-level pressure transducer must be used to withstand the high average or static pressure (which may also be of measurement interest), and the small variation to be monitored results in only a tiny variation in the average output signal. These minute variations become buried in the noise inherent in the measurement system, limiting the ability of the transducer system to provide a useful output signal reflecting the target small pressure changes.
The variable-reluctance transducer of this invention provides a solution to this problem, and is described in terms of a pressure transducer, though the principles of the invention are applicable to accelerometers and other styles of transducers. Broadly, the concept is to provide a variable-reluctance system which generates a very high output signal which lifts the desired small output variations above the relatively constant noise level of the associated electronics so the small output variations can be detected and recorded or displayed.